Idea: Can declaration emit synthesize type aliases?

[amcasey]

Object types can be very large and in (e.g.) a react project that makes heavy use of type inference, it's not uncommon to have to refer to a type that is either not imported or is simply anonymous (e.g. a props type). When these appear in multiple places in a declaration file, they tend to expand it considerably. Perhaps we could just synthesize type aliases for them?

• We'd have to be careful not to change the exported API shape
• We could use a heuristic to skip the work for small types (e.g. < 20 characters)
• Even if we didn't wait until the second use of a type to decide to extract it, we'd be adding ~4 tokens per type (type id = and id at the use site)

[amcasey]

With #44044, this is part of a larger discussion of how we could reduce the size of declaration files to make them faster to read and write (ideally, without sacrificing readability).

[weswigham]

#30979 is along the lines of what I think we'd prefer to see

[amcasey]

@weswigham What's the scope of those inline type aliases? Being able to use them across multiple declarations seems key to reducing declaration file size.

[weswigham]

In that case, within a given type node, so you can serialize a recursive or self-referential type. (Which today usually expand till they hit a depth limiter)